1. Field of the Invention
The present invention relates to a pickup actuator employed into an optical device for recording/reproducing information on/from optical disc by using an optical means.
2. Description of the Related Art
A field of optical means for recording/reproducing an information on/from information recording medium such as optical disc is already in the limelight as a new technique of the coming era. Especially, a pickup actuator is considered as an important component of the optical device, since it determines the speed of recording/reproducing of information. Such a pickup actuator schematically comprises a base member, a lens holder, and lens holder driving means. The lens holder is movably disposed with respect to the base member. An objective lens is mounted on the lens holder. The objective lens projects a light beam to a recordable side of the optical disc to read the information as an optical signal, or record the information on the optical disc. The lens holder driving means drives the lens holder toward directions of focusing and tracking.
An example of such a pickup actuator is shown in FIGS. 1 to 3.
FIG. 1 is an exploded perspective view showing a conventional pickup actuator, FIG. 2 is a perspective view showing the pickup actuator assembled, and FIG. 3 is a sectional view for showing a main portion of the pickup actuator shown in FIG. 2.
A reference numeral 10 is a base member. As shown in FIGS. 1 to 3, the base member 10 comprises a body portion 12 and a holder supporting portion 14. A pair of yokes 20 and 20' are disposed at an upper side of the body portion 12, while facing each other and having a predetermined distance therebetween. The holder supporting portion 14 is attached to an outer side surface of one of the yokes 20 and 20' by a plurality of screws 16a and 16b.
Further, a pair of magnets 30 and 30' are attached to respective inner side surfaces of the yokes 20 and 20'. The magnets 30 and 30' form a magnetic circuit. The yokes 20 and 20' function to maximize and concentrate the magnetic flux density toward a desired direction.
A reference numeral 40 is a lens holder. The lens holder 40 is disposed between the magnets 30 and 30' which are disposed at the base member 10. An objective lens 42 is mounted on the lens holder 40. The lens holder 40 is suspended at the holder supporting portion 14 of the base member 10 by two pairs of wire springs 44a and 44b such that the lens holder 40 is movable. One ends of the wire springs 44a and 44b are connected to both sides of the lens holder 40, while other ends thereof are connected to the holder supporting portion 14 after passing through a space 14a defined at an inner portion of the holder supporting portion 14.
Further, a driving coil 50 is disposed at the lens holder 40 together with the magnets 30 and 30', constituting a lens holder driving means.
The driving coil 50 has a pair of focusing coils 52a and 52b and a tracking coil 54. The focusing coils 52a and 52b are wound around opposite sides of the lens holder 40, while the tracking coil 54 is wound in a perpendicular fashion with respect to the focusing coils 52a and 52b. The focusing coils 52a and 52b and the tracking coil 54 move the lens holder 40 toward directions of focusing and tracking, in cooperation with the magnets 30 and 30'.
In a conventional pickup actuator constructed as above, focusing and tracking of the objective lens 42 are performed by an electromagnetic interaction between the magnets 30 and 30', the focusing coils 52a and 52b, and the tracking coil 54.
The above conventional pickup actuator, however, has a drawback as follows. Since the lens holder 40 is suspended at the holder supporting portion 14 by the wire springs 44a and 44b, during focusing and tracking of the objective lens 42, a lot of time is taken for the objective lens 42 to reach the designated spot and be settled therein.
The above drawback will be described in greater detail with respect to FIG. 4. As shown in FIG. 4, an Y-axis ("S") is a distance that the objective lens 42 is supposed to be moved to be settled in the designated spot, and an X-axis ("t") is a time taken for the objective lens 22 to reach the designated spot and be settled therein.
"Ga" in FIG. 4 is a line for indicating an ideal movement of the objective lens 42. In this case, it takes "0" (zero) seconds for the objective lens 42 to reach the designated spot "T" and be settled therein. However, it is almost impossible because the objective lens 42 has to have a speed reaching an infinite value in an instant, to be moved to the designated spot "T".
"Gb" in FIG. 4 is a line for indicating an actual movement of the objective lens 42. In this case, it takes "t.sub.b " for the objective lens 42 to reach the designated spot "T" and be settled therein.
If the objective lens 42 is moved at a greater speed to shorten the time "t.sub.b ", the time can be shortened to "t.sub.c ", however, there inevitably occurs the vibration of the objective lens 42 until the objective lens 42 is settled in the designated spot "T".
As described, the greater the speed the moving objective lens 42 has, the greater vibration the objective lens 42 has. Thus, if the vibration is efficiently damped, the objective lens 42 can be settled in the designated spot "T" more rapidly.
In the past, it has been suggested that a damper bond 60 in a gel state be injected into the inner portion of the holder supporting portion 14 in order to damp the vibration. Due to the presence of the damper bond 60, the vibration transmitted through the wire springs 44a and 44b is damped more rapidly, namely in a time "t.sub.d ". (Ref. "Gd" line in FIG. 4)
Despite the damper bond 60, however, there is a limit to increasing the speed of the focusing and tracking operation of the objective lens 42, since the vibration occurring at the lens holder 60 is damped by the damper bond 40 only after the vibration is transmitted through the wire springs 44a and 44b to the damper bond 60.
Particularly when the optical device is employed into the automobiles which have intensive outside vibrations, or when the physical shock is exerted to the optical device, the vibration occurring at the lens holder 40 is not quickly damped, and there occurs a mis-function of the optical device.
Further, the damper bond 60 has to be injected into the inner space 14a defined within the holder supporting portion 14 from a side thereof. Thus, injecting of the damper bond 60 itself has been a complex process, which deteriorates a productivity of the optical pickup device.
In addition, after injecting the damper bond 60, the damper bond 60 has to be solidified to some degree by ultraviolet rays radiated thereto. Thus, a manufacturing process of the optical pick device becomes more complex, and a manufacturing cost thereof accordingly increases.